With the background of the rapid spread of office automation, high-quality or sharp, light-transmissive, well-fixed copied images have been in higher demand than before in electrostatically charged image developing copiers and printers.
One of requirements for such high-quality images is prevention of an offset phenomenon that a toner is not completely fixed on a copying medium such as paper or a film and a part thereof remains on a heat roller and is fixed on a next copying medium to stain the same.
A heat (fixing) roller of a copier is usually heated at 160 to 180.degree. C. in operation. Accordingly, a toner is designed in order to be fixed (bound) at 120 to 190.degree. C. by providing an extra width. The offset phenomenon occurs when a temperature is too high or too low. Thus, for preventing the offset phenomenon and improving the fixability and the sharpness, it is important to secure a toner fixing temperature range which does not cause the offset phenomenon, namely, the offset-free temperature range as broad as possible within said range of 120 to 190.degree. C.
Under these circumstances, the present inventors tried to improve the fixability by selecting a high-viscosity olefin polymer having a cyclic structure as a binder resin for a toner, and further proposed that waxes having a melting point of 60 to 170.degree. C. are added for preventing the offset phenomenon (refer to JP-A-101631/97) . However, the offset-free temperature range so far realized could not reach to the level to be altogether satisfactory enough.
Moreover, the inventors proposed in International Patent publication No. WO 98/29783 specification that the high-viscosity olefin polymer having a cyclic structure is selected as a binder resin for a toner for prevention of the offset phenomenon and a wax selected from amide wax, carnauba wax, higher fatty acids and their esters, higher fatty acid metallic soaps, partially saponified higher fatty acid esters, higher fatty alcohols, polyolefin waxes and paraffin waxes is used. The offset-free temperature range was in the range of 30 to 40.degree. C., and the binder resin could be put to practical use. However, a satisfactory offset-free temperature range being applicable when a binder resin is an olefin polymer having a cyclic structure and a combination of different molecular weights or is made of plural different polymers has not been obtained.